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Sudden Sinkhole
Photograph from AFP/Getty Images
Onlookers in Beijing, China, keep a safe distance from a giant sinkhole that opened in the middle of a busy street last Tuesday, swallowing a truck. Several news reports say the sinkhole formed above a tunnel being dug for construction of a subway line. (See pictures of a huge sinkhole in Guatemala City.)
But Chinese engineers urged caution in jumping to conclusions: "It could have resulted from multiple causes," Wei Jinglian, a senior engineer from the Beijing Institute of Geology, told China's Global Times. "Water leakage from old pipes underground may have softened the soil," Jinglian said, "or there has been too much pressure frequently on the same section of road."
Mining and construction activities can weaken the subsurface and cause sinkholes to collapse, said Mike Hoyal of the Tennessee Division of Geology. But sinkholes can also form naturally when water-saturated soil becomes too heavy, causing the roofs of existing voids and caves in the ground to collapse.
In addition, acidic rain or groundwater can enlarge a natural fracture in a limestone bedrock layer to form a sinkhole. The water "dissolves out the calcium carbonate in the limestone and forms fractures," Hoyal said. As the crack gets bigger, the topsoil can gently slump, exposing the sinkhole to the world.
—Ker Than
Published May 3, 2011
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Narrow Escape
Photograph from AFP/Getty Images
The tail end of a truck protrudes from debris inside the Beijing sinkhole on April 26. According to news reports, the driver and a passenger jumped out of the vehicle before it sank into the hole. Both people were only slightly injured.
Engineers typically conduct foundation studies to test the stability of the ground at a site before beginning any major construction, such as a city highway. But "that's only been true within the last few decades," Hoyal said. It's possible, he said, that the Beijing sinkhole formed after a new roadway was built on top of an older road that did not have foundation studies done before its construction.
Based on the pictures, Hoyal estimates that the Beijing sinkhole is about 25 feet (7.6 meters) wide and about 35 feet (10.6 meters) deep.
"That's a pretty impressive sinkhole," he said. (See pictures of sinkholes around the world.)
Published May 3, 2011
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Road Repairs
Photograph from AFP/Getty Images
Construction crews begin to repair the giant Beijing sinkhole, which formed the night of April 26.
It can take thousands of years for the pit of a sinkhole to form naturally beneath Earth, Hoyal said, but the collapse of the ground above can be instantaneous. Over time "the soil cover and limestone cap rock [above a sinkhole] get weakened, and all of a sudden the bottom drops," he said.
Published May 3, 2011
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Filling in the Gap
Photograph from AFP/Getty Images
Excavators work to repair the Beijing sinkhole in a picture taken last week.
Weather can contribute to the formation of sinkholes, Hoyal said. For example, during an extended drought, natural clays in the ground can form cracks, shrink, and become "less sticky."
When rain falls on the region again, or if underground water seeps up toward the surface, "the clays in the soil expand and lose their cohesive properties, and the soil becomes unstable ... and it causes structures to fall," Hoyal said.
Published May 3, 2011
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